The invention relates generally to the field of circuitry which produces an output signal having an adjustable amount of phase with respect to an input signal. The invention more particularly relates to the use of such an adjustable phase circuit in the ignition spark timing circuitry for an internal combustion engine.
In electronic ignition systems for internal combustion engines, it is generally desirable to produce a spark timing ignition signal at the spark plugs of the engine at a fixed time with respect to the rotational position of the crankshaft of the engine. Typically, a reluctor is synchronously rotated with respect to the crankshaft of the engine and a stationary magnetic pickup coil is used to produce an AC signal related to the rotational movement of the crankshaft of the engine. This AC signal is conventionally coupled to a transistor or other type of semiconductor device, and a substantially digital logic signal which has a frequency related to the speed of the engine and leading and trailing edges which have a precise relationship to the rotational position of the crankshaft is produced. This digital logic signal is subsequently used to produce spark plug ignitions at predetermined angular positions of the crankshaft. The term "leading edge", as used herein, refers to a low to high logic state transition of a digital signal, while the term "trailing edge" refers to a high to low logic transition.
The digital logic signal produced by the sensor is used to produce a spark timing signal having an adjusted phase to provide for the proper time occurrence of the spark plug ignition with respect to the rotational position of the crankshaft and various engine parameters, such as engine speed and engine vacuum pressure. Generally, the phase adjustment is accomplished by adjusting the time occurrence of either the leading or trailing edge of a timing signal which corresponds to the digital logic sensor signal. One such prior art system is illustrated in a copending U.S. patent application, Ser. No. 537,726, by Chi Sun Lai and Philip Gunderson, which is entitled "An Electronic Spark Timing Adjustment Circuit" and which is assigned to the same assignee as the present invention. In this prior art system the leading or trailing edge of a digital sensor signal is adjusted by a controllable predetermined amount of phase so as to produce a desired spark timing signal. The prior art system is satisfactory for providing a time occurrence adjustment for the occurrence of a spark plug ignition. However, this signal edge adjustment technique commonly can result in altering the dwell (coil excitation time) for the ignition system. Such a result is obviously not desirable since the dwell time should be independent of the spark timing adjustment. Generally, prior art ignition systems have not provided any simplified circuitry which would provide for an adjustable total phase shifting of a signal whereby both the leading and trailing edges of the signal are uniformly shifted by an adjustable predetermined amount of phase.